Thermocouple



E. c. scHUNKE THEM/xocomme` Filed July 2s, 195s www INVENTORTHERMOCOUPLE Edwin C. Schunke, Speedway, Ind., assignor to GeneralMotors Corporation, Detroit, Mich., a corporation of DelawareApplication July 23, 1953, Serial No. 369,856

4 Claims. (Cl. 136-4) This invention relates to thermocouple probes andmore particularly to shielded sampling probes for measuring thetemperatures of highly heated, high velocity gases flowing through ductsof relatively lower temperature.

When a thermocouple is placed in a hot, high velocity gas duct such asthe inlet or outlet duct of a gasturbine, the E. M. F. produced therebyaccurately measures the temperature difference between the hot an-d coldjunctions of the thermocouple but not necessarily the actual temperatureof the gas. Heat transfer from the highly heated gas to the hot junctionis primarily by convection while heat transfer from the hot junction tothe lower temperature duct is by radiation and conduction. Heat transferto the junction must proceed at the same rate as heat transfer from thejunction in order to obtain an accurate indication of the gastemperature. Heat transfer from the junction tends to be excessiveunless special precautions are taken in the design of the gas probe.

Heat transfer from the junction by conduction may be reduced by placingthe junction as far as possible from the duct wall and/or by exposingmost of the thermocouple Wire directly to the hot gas stream. This meansof reducing conduction heat transfer is impractical in applicationswhere the size of the duct limits the distance that the junction may beplaced from the duct wall and/ or where the gas stream is of such highvelocity that a casing and insulation is required to reinforce thethermocouple wires up to the junction to prevent their bending from theimpact of the gas stream. Heat transfer from the junction by radiationmay be reduced by providing areflective shield between the junction andthe duct Wall. Conventional insulated gas probes are usually designed tohave a length to diameter ratio of to 1 to reduce conduction heattransfer from the junction and are shielded in some instances to reduceradiation heat transfer from the junction so that the heat transfer toand from the junction will be in equilibrium, but these probes areunsuitable for measuring the temperature of high velocity gases in smallducts, such as the nozzle inlet duct of a gas turbine, because of theirexcessive length. Another consideration in gas probe design results fromtemperature differentials between various gas strata in the duct makingsome form of sampling desirable to obtain an average gas temperature.

An object of the invention is to provide an insulated and reinforced gasprobe of simple and sturdy construction and of minimum length todiameter ratio wherein heat transfer between the junction and anassociated gas duct wall will be in substantial balance.

A further object of the invention is to provide such a gas probe withmeans for averaging the temperatures of various gas strata in a gasduct.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawings wherein a preferred form of the present invention is clearlyshown.

In the drawings:

Fig. 1 is a partial longitudinal section through the tates Patent"annular intake duct of a gas turbine or the like showing a partiallybroken away thermocouple according to the invention mounted therein;

Fig. 2 is a partial longitudinal section through the annular intake ductof a gas turbine or the like showing a partially broken awaythermocouple according to a different embodiment of the inventionmounted therein;

Fig. 3 is a section taken substantially on the plane indicated by theline 3 3 of Fig. 2, and

Fig. 4 is a section taken substantially on the plane indicated by theline 4-4 of Fig. 1.

Referring now to the drawings, only so much of the intake duct of thegas turbine is shown as is necessary for an understanding of theinvention. The duct comprises a cylindrical outer wall 10 and acylindrical inner wall 12 forming an annular passage for the ow ofhighly heated, high velocity gas in the axial directions indicated bythe arrows A, B and C. The duct walls are exposed on their exterior tocooling air making them of lower temperature than the gases therein.

Referring now to Figs. 1 and 4 for the preferred embodiment of theinvention, the gas probe 14 is suitably secured to the outer wall 10 bya flange 16 and includes a stainless steel tubular shield 18 whichprojects radially into the gas passage, that is, normal to the flow ofthe gas strata A and B. A pair of stainless steel tubes 20 and 22 aresecured in spaced relation in the shield 1S by welds 24 and 26 to forman elongated chamber 28 in the shield. Two pairs of Chromel-Alurnelthermocouple wires 3@ and 32 are supported in the tubes 20 and 22 bymagnesium oxide insulation 34 and 36 packed therein. The thermocouplewires 30 are bent together and welded to form a hot thermocouplejunction ill in the center of the chamber 2S, a similar welded junction42 being provided for the thermocouple Wires 32. Only one thermocouplewire pair is necessary, the extra thermocouple wire pair being providedin the interest of safety or for transmitting an E. M. F. to a differentindicating instrument. Gas inlets 44 and 46 are provided at each end ofthe chamber 28 to intake gas from the separated strata indicated by thearrows A and B pass the gas over the exposed substantial length ofthermocouple wires and the junctions for subsequent discharge out adiametrically opposed central outlet 48. The shield 18 and theinsulators 34 and 36 provide support for the thermocouple wires at eachend of the chamber 28 to reinforce them from bending under the impact ofthe gas flow through the chamber thereby permitting a substantial lengthof thermocouple wire to be exposed in the chamber so that conductionheat transfer from the junctions to the duct wall 10 is reduced. Thisresults in an accurate insulated gas probe having a smaller length todiameter ratio than those previously known. The shield 18 reducesradiation heat transfer from the junctions to the duct wall 10 and thespaced inlets 44 and 46 and central outlet 48 provide gas from theseparated strata A and B to the centrally located junctions so that anaverage temperature of the gas in the duct is obtained.

Referring now to Figs. 2 and 3 for the other embodiment of theinvention, the gas probe 50 is threaded through the outer duct wall lllat 52 to project normal to the flow of the gas strata A, B and C in theduct. The thermocouple Sli includes a stainless steel tubular shield 54having magnesium oxide insulators 56, 58 and 60 packed therein andspaced from each other to form chambers 62, 64 and 66 at increasingdistance from the wall 10. A first pair of Chromel-Alumel thermocouplewires 68 are supported in the shield 54 by the insulators 56 and 58 andextend exposed through the chamber 62 and into the chamber 64 where aweld 70 provides their hot junction. A second pair of Chromel-Alumelthermocouple wires 72 are supported in the shield 54 by the PatentedJan. 21, 1958- 3 insulators "56, 58iand60 Yand extend exposed throughthe chambers 62`and' 64 andv into the'chamber 66 for hotl junctionjoinder by a weld 74. The chambers 62, 64 and 66 are provided withdiametrically opposed inlet and outlet` openings 76v and'78), Sitiand'82, and S4 and 86` forthe ow of gas from'the respective stratas C, Band A through the chambers. Theil'owof gas through1the chamber 62`overthe exposed substantial lengthofthermocouple wire therein reducesconduction heat transfer from the junctions 7@ and '74 to the wallllt)while the shield 54` and the respective insulators 58 andy 68' beyondvthe exposed length of thermocouple wire reinforce the same againstbending by the impact ofthe gas flow' through the chamber. Radiationheat transfer from the junctions to the wall itl is reduced by theshield 54.' The litre leads ofeach thermocouple pair 68and72"areconnected together inparallel'at the junction posts 88 and 98 andtheY pairs are of dilferent diameter so that each has the'same`radiation heat transfer to the wall`and sample separated gas strata toachieve an average gas temperature indication.

While the preferred embodiment of the invention has been described fullyin order to explain the principles of the invention, it is to beunderstood that modifications in structure may be made by the exerciseof skill in theV art within the scope of the invention, which is not tobe regarded as limited by the detailed description of the preferredembodiment.

I claim:

1. A temperature probe for gases comprising a casing, insulators in thecasing spaced from each other to form a chamber therein, and a spacedpair of thermocouple wires in the chamber being free of contact with thecasing and having a junction with each other located between theinsulators, each wire extending the full length of the charnber andbeing supported at each end of the chamber by the insulators, thecasinghaving inlet and outlet openings for gasflow through the chamber.

2'; A temperature probe for gasescomprising' a casing,v insulators inthe casing spaced from each other to form a chamber therein, and aspaced pair of thermocouple wiresk in the chamber being free of contactwith the casingand having a junction with eachother located centrally inthe'chamber, each wire extending the full length of the chamber andbeing supported at each end of the charnber by the insulators, thecasing having an inlet opening at each end of the chamber for gas owthrough the chamber and an outlet opening at the center of the chamber.

3.* A'gasprobe comprising a shield adapted to be supported by a gas ductwall, the gas in the duct having a different temperature than that ofthe duct wall, insulators in the shield spaced from each other to form achamber therein, and a spaced pair of thermocouple wires in the chamberbeing free of contact'with the shield and having a junction with eachother between the insulators, each wire being supported at each end ofthe chamber by the insulators,the` shield beingV effective to reduceradiation heat transfer between the wall and junction, the thermocouplewires being exposed over that portion of their length inthe chamber, theshield having an inlet opening at each end of the chamber and an opposedoutlet opening at the center of `the-chamber for the flow of gas throughthe shield and across the junction and exposed portion of wire length,the flow of gas across the exposed portionof wire lengthbeing effectiveto reduce conduction heat transfer between the wall and junction.

4. A gas probe comprising a metallic tube adapted to be supported by aduct wall normal to the ow of a gas in the duct at a differenttemperature than that of the duct wall, insulators in the tube spacedfrom each other to form an elongated chamber therein, and a spaced pairof thermocouple wires in the chamber being free of contact withthe tubeand having a junction with each other located centrally therein, eachwire being supported at each endlof thechamber by the insulators, thetube being effective to reduce radiation heat transfer between the walland junction, the thermocouple wires being exposed over that portion oftheir length in the chamber, the tube having an inlet`opening facingupstream at each end of the chamber and a diametrically opposed outletopening at the center of the chamber for the ow of gas from differentstrata through the tube and across the junction and exposed portion ofwire length, the tube openings and chamber being effective to obtain anaverage gas temperature, the flow of gas across theexposed portion ofwire length being effective to reduce conduction heat transfer betweenthe wall and junction, the tube and insulators being effective toreinforce the thermocouples wire against bending by the impact of. thegas flow.

References Cited in the le of this patent UNITED STATES PATENTS1,526,641 Mulvany et al. Feb. 17, 1925 2,246,563l Winters June 24, 19412,314,877 Hall Mar. 30, 19,43 2,410,218 Kelly Oct. 29, 19,46, 2,485,468lWrigleyl Oct. 18, 1949 2,496,806 Moffatt Feb. 7, 1950 2,496,835 WardFeb. 7, 1950 2,652,723 Hastings Sept. 22, 1953 2,653,983 Best Sept. 29,1953 FOREIGN PATENTS 461,370 Great Britain Feb. 16, 1937

